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List:       fedora-devel-list
Subject:    Re: Enable EarlyOOM on Fedora KDE - Fedora 33 Self-Contained Change proposal
From:       Benjamin Berg <bberg () redhat ! com>
Date:       2020-07-18 9:35:14
Message-ID: 586d41b3a751fa7c1ef51f79113d72067851f724.camel () redhat ! com
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On Sat, 2020-07-18 at 05:07 +0900, Alexey A. wrote:
> > And this will turn bad quickly, as it will eventually
> > include the executables/libraries that must be loaded as they are doing
> > work!
> 
> > So, we don't want to get the kernel into the situation where it must
> > remove executables/libraries from main memory. If that happens, you can
> > end up hitting the disk for *every* function call.
> 
> BTW, with prelockd[1] executables/libraries will not be removed from
> memory. This daemon can mlock executables/libraries in memory.
> 
> Effects:
> - OOM killer comes faster.
> - Fast system reclaiming after OOM.

Interesting, I had not seen that.

I wonder how much memory that amounts to in the usual scenarios. Could
be interesting to compare this to the point where EarlyOOM or similar
would kick in.

That said, I am sure that it is really effective at preventing many
really bad thrashing scenarios.

Benjamin

> This daemon was written recently, published today.
> 
> 1. https://github.com/hakavlad/prelockd
> 
> вт, 14 июл. 2020 г. в 17:19, Benjamin Berg <bberg@redhat.com>:
> > On Mon, 2020-07-13 at 19:25 -0600, Chris Murphy wrote:
> > > On Mon, Jul 13, 2020 at 12:00 PM Benjamin Berg <bberg@redhat.com> wrote:
> > > > If MemAvailable drops below 250MiB (roughly 6GiB * 4%), then this means
> > > > that we have less than 500MiB left for file caches. If we can't swap
> > > > (remember, swap is already pretty full too), then a big chunk of these
> > > > caches need to be dropped to make the memory available to applications.
> > > 
> > > I regularly see impressively low MemAvailable before earlyoom does
> > > SIGTERM, it's almost always swap available (amount of total that's not
> > > used) that's the defining factor. Well below 100MiB. This doesn't tend
> > > to last very long. Once memory is under this kind of pressure, so is
> > > swap, and as swap on zram is so fast, it gets to threshold fast as
> > > well.
> > 
> > Yep, EarlyOOM doesn't apply the limit if there is swap available. That
> > makes a lot of sense. When swap page is available, the kernel has the
> > choice which memory to reclaim (anonymous pages, i.e. swap, or file
> > backed pages, i.e. drop caches). So MemAvailable may drop much lower
> > temporarily and depending on the workload.
> > 
> > You could say that when swap is available you assume the kernel has the
> > ability to keep the system running reasonably well. Once swap runs out,
> > the kernel stops having a choice. It can only make room by reclaiming
> > important caches. And this will turn bad quickly, as it will eventually
> > include the executables/libraries that must be loaded as they are doing
> > work!
> > 
> > So, we don't want to get the kernel into the situation where it must
> > remove executables/libraries from main memory. If that happens, you can
> > end up hitting the disk for *every* function call.
> > 
> > Benjamin
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